1. Field of the Invention
This invention relates to semiconductor devices, and in particular to techniques for stabilizing and enhancing breakdown voltage of semiconductor devices.
2. Description of the Background Art
It is generally required that power semiconductor device have high capability of holding breakdown voltage, namely, stability improvements in and enhancements of breakdown voltage are desired. Techniques known to accomplish this objective include a field plate, a guard ring, and a structure combining those elements (e.g. Japanese Patent Application Laid-Open No. 2003-188381). In another technique, diodes that are connected back-to-back with one another (back-to-back diode) are disposed between the gate and collector of an IGBT (insulated gate bipolar transistor) to thereby prevent an overvoltage from being applied between the collector and emitter (e.g. Japanese Patent No. 3191747 (2001)). Also known are a semiconductor device structure combining the back-to-back diode and guard ring (e.g. Japanese Patent No. 3331846 (2002)), and a semiconductor device structure combining the back-to-back diode and field plate (e.g. Japanese Patent Application Laid-Open No. 10-163482 (1998)).
A field plate structure is generally capable of achieving a high breakdown voltage of a semiconductor device with a small area. However, being formed on a substrate, a field plate is susceptible to polarization of a resin and the like for molding a chip top surface (mold polarization), which leads to breakdown voltage fluctuations and so on to cause the breakdown voltage to be unstable. A guard ring structure, although being capable of obtaining a stabilized breakdown voltage compared to the field plate structure, needs a larger forming area than that for the field plate structure. Besides, in a semiconductor device structure combining a back-to-back diode and guard ring, a potential difference develops easily between a region below the back-to-back diode and the remaining region in the guard ring, which leads to an unstable breakdown voltage.
In the above JP 10-163482 (1998), a group of ring-shaped field plates (equipotential rings) are formed via an insulating film on a silicon substrate around an IGBT (insulated gate bi-polar transistor) forming region on an IGBT chip, and belt-shaped back-to-back diodes are disposed in a ring shape between each of the equipotential rings to connect between the rings. In short the back-to-back diodes are formed on the whole periphery of the chip, giving a problem of an increase in forming area for the field plate structure. Moreover, since the directions of the back-to-back diodes become the one along each of the equipotential rings, the potentials at the back-to-back diodes vary to the direction along the chip periphery. This makes the potential at the chip peripheral portion unstable, which results in an unstable breakdown voltage easily.
Also in the IGBT disclosed in the JP 10-163482 (1998), the back-to-back diodes break down to pass a current between the gate and collector when the voltage between the collector and emitter exceeds a prescribed value. Consequently, a gate electrode potential increases to render the IGBT into the ON state (conduction state), thereby preventing an overvoltage from being applied between the collector and emitter. The back-to-back diodes thus need to have a certain degree of width for passing the breakdown current. However, the above problems (increase in forming area for the field plate structure, and instability of the potential at the chip peripheral portion) become more pronounced with the widening of the diodes, and a too much widening will make it difficult to maintain a high breakdown voltage or will increase a leakage current. Therefore, there is a maximum width of the back-to-back diodes, which constitutes a restriction on chip design.